Thermal Capacity of Railway Wheels - Temperatures, residual stresses and fatigue damage with special focus on metro applications

Doctoral thesis, 2014

Tread (block) braking is still one of the most common braking systems on railway vehicles. The action is carried out by pressing brake blocks against the tread of a wheel, which is also in rolling contact with the rail. The extensive use of tread brakes in metro and suburban applications has created a need for design guidelines or standards for wheels exposed to repeated stop braking. The thermal capacity of the wheels puts a limit to railway tread braking systems. With the exception of the drag braking cases described in the European standard EN 13979-1, there are no known standards or guidelines regarding the thermal capacity limits for wheels. In the present work, important aspects of the thermal capacity of tread braked railway wheels have been assessed in a literature survey. Then two different railway wheel designs, with typical characteristics of freight and metro wheels, have been numerically studied with respect to standard design criteria for load cases of drag braking and stop braking. The influence of brake block materials, thermal parameters and brake pressure distribution on the wheel temperatures has been investigated. A general result is that hot spots only have a minor influence on the global heat partitioning in the wheel-block-rail system even though the hot spots have a major impact on local temperatures. Brake rig experiments and a field test campaign were performed and aimed at measuring wheel and brake block temperatures during different service conditions for a metro line. Simulation and calibration tools were employed in order to facilitate a comparison between measured temperatures. The results showed the importance of knowing the convection cooling parameters for different wagons if prolonged braking action is to be considered. In a pin-on-disc experimental study of railway braking materials, the heat partitioning characteristics between wheel and block material at controlled elevated disc temperatures were investigated by a finite element approach where a model was calibrated using measured temperatures. In the final part of the present thesis, a modelling framework was proposed and developed that represents typical conditions in metro and suburban operations, in particular during sequential stop braking. A parametric study was done for analysing the influence of various loading levels and other important factors on temperatures, axial flange deflection, residual stresses and the fatigue life of the wheels. The model and the numerical results will be useful for assessing the thermal capacity of wheels and for developing new design rules and standards. It was found that the mechanical and thermal loadings have different influences on the web damage and on the estimated fatigue life depending on load cases and wheel design.